In recent years, GaN-based compound semiconductor materials have been attracting attention as semiconductor materials for short-wavelength light emitting devices. GaN-based compound semiconductors are fabricated on substrates of various kinds of oxides and III-V compounds, including single-crystal sapphire, by using such methods as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
One of the characteristics of GaN-based compound semiconductor materials is that current diffusion hardly occurs in lateral directions. As a result, the current is injected only into the semiconductor directly below the electrode, and the light emitted from the light-emitting layer is blocked by the electrode and cannot be extracted outside. One solution is to form the positive electrode of this type of device from a transparent electrode so that the light can be extracted through the positive electrode. Another solution is to employ a flip-chip design which uses a reflective positive electrode and allows the emitted light to be extracted through the substrate.
The positive electrode of a conventional flip-chip device is formed in a layered structure by combining a contact metal, such as Pt or Ni, and a reflective metal, such as Rh or Ag (for example, refer to Japanese Unexamined Patent Publication No. 2000-183400).
On the other hand, the external quantum efficiency of a light-emitting device is defined as the product of the light extraction efficiency and the internal quantum efficiency. The internal quantum efficiency indicates what percentage of the electric current energy injected into the device turns into light. The light extraction efficiency indicates what percentage of the light generated within the semiconductor crystal can be extracted outside.
In the solution that uses a transparent electrode, one technique employed to increase the efficiency of extracting light from the semiconductor crystal into the atmosphere is to form depressions/protrusions on the light extraction surface of the semiconductor. Methods such as dry etching, wet etching, dicing, scribing using a diamond stylus, and laser scribing can be employed to form depressions/protrusions on the semiconductor surface. However, when the semiconductor material is subjected to such processing, the semiconductor layer suffers damage because of the strain due to the processing; as a result, even if the light extraction efficiency is increased by such processing, the internal quantum efficiency decreases, ending up being unable to increase light emission intensity. Furthermore, there have also been cases where the light-emitting device breaks down due to leakage current, etc., leading to low production yields.
In view of the above situation, there is proposed a technique for enhancing the light extraction efficiency wherein a layer to be formed with the depressions/protrusions for increasing the light extraction efficiency is formed on the semiconductor layer (for example, refer to Japanese Unexamined Patent Publication No. 2000-196152). According to this technique, since the depressions/protrusions are formed, not on the semiconductor layer itself, but on the transparent material layer formed on the semiconductor layer, the light extraction efficiency can be increased without causing damage to the semiconductor. However, what is described in this Japanese Unexamined Patent Publication No. 2000-196152 is an invention that specifically concerns a device of the type that uses an optically transmissive electrode. This Japanese Unexamined Patent Publication No. 2000-196152 also discloses polycarbonate, silicon nitride, indium oxide, niobium oxide, antimony oxide, zirconium oxide, cerium oxide, titanium oxide, zinc sulfide, bismuth oxide, etc. as examples of the transparent material used to form the layer on which the depressions/protrusions are to be formed.
In the case of a thin-film layered structure such as a conventional semiconductor device, multiple reflections are one of the factors working to reduce the light extraction efficiency. That is, multiple reflections occur at such surfaces as the front and back surfaces of the substrate, the interface between layers having different dielectric constants, and the surface where a reflective film is formed, and the emitted light is attenuated as it travels through the transparent material, due to absorption in the material, etc.
In the case of a flip-chip, multiple reflections occur at the reflective electrode and the interface between the semiconductor and the substrate, and these multiple reflections reduce the light extraction efficiency. There is therefore a need to construct a structure that avoids such multiple reflections by forming depressions/protrusions on either one of the reflecting surfaces.
One method is to form depressions/protrusions on the interface between the substrate and the semiconductor by forming the depressions/protrusions on the substrate on which the crystal is to be grown (for example, refer to Japanese Unexamined Patent Publication No. 2002-164296). With this method, however, since the substrate on which the crystal is to be grown has to be formed with depressions/protrusions, it becomes difficult to form a clean mirror-like crystal film uniformly and stably within the wafer. Another method is to form depressions/protrusions on the reflective electrode surface (for example, refer to U.S. Pat. No. 6,563,142). In the prior art flip-chip electrode, however, since either the contact metal is made to also serve as the reflective metal or the contact metal is extremely thin, the semiconductor surface has had to be processed in order to form the depressions/protrusions on the reflective electrode surface. As earlier described, if the semiconductor surface is subjected to such processing, the internal quantum efficiency degrades, and the light emission output cannot be increased as desired.